1. Field of the Invention
The present invention generally relates to clock systems for computers and, more particularly, to an optical clock system wherein a train of ultrashort optical pulses are generated and transmitted in parallel through optical delay lines to provide optical pulse trains with different desired delay times, and a receiver capable of detecting and converting these pulses into the appropriate electrical signals. The invention has particular application in very high performance computer systems employing very large scale integration (VLSI) and wafer scale integration (WSI) technologies.
2. Description of the Prior Art
Clock timing is a critical factor in digital computing systems. As machine cycle times become shorter, clock skew, which is the temporal uncertainty in clock signals, becomes an increasingly larger fraction of the total cycle time. For systems whose cycle times are below four nanoseconds (ns), clock skew is becoming a major limiting factor.
Since functions in a digital computing system are coordinated by a master clock, accurate distribution, synchronized with the arrival of various signals, must be tightly controlled. Presently, electrical clock signals are transmitted to the various circuit modules in a computer system. Uncertainties in the electrical distribution of signals are due to variations in propagation time through passive and active circuit elements, as well as the influence of coupled noise sources. Passive elements are electrical cables, board and substrate transmission lines, connectors, bonding elements and through vias. Active circuit skew is caused by the difference in propagation time between circuit elements, threshold variations, etc. due to variances in circuit temperature, loading and power supply voltage. Major contributions to skew are the uncertainty of the propagation time and waveform distortion in the electrical transmission path between active circuit elements.
Several proposals have been made to reduce clock skew in electronic computing systems by introducing optical means of transmitting clock information from an electrical clock generator to the appropriate receiving device. Most of these approaches can be considered to be an optical transmission parallel to the present electrical clock distribution system. They are generally based on using a standard electrical oscillator (clock generator) to drive a laser. Typically this laser is a semiconductor diode laser. These approaches are limited because they require powerful laser diodes as well as a fast, clean response. These are difficult to achieve.
Another system of optical clock distribution has been proposed that uses line-of-sight broadcasting of the clock signals directly to the receivers. In this system, there would be a limited number of clock signals (usually just one) that may be transmitted, and since the energy is distributed over a large area, it would require very large optical power to operate.